This invention pertains to the field of electrical power and energy distribution and conversion in large structures such as high-rise buildings, office complexes, factories, ships, large airplanes, etc. providing a higher efficiency than the existing system thus saving expensive energy.
It is believed that xe2x80x9crelated background artxe2x80x9d does not exist. To the best of the inventors knowledge nobody has described or built an energy/power distribution system based on the integration. Background art in itself is presented by the existing energy/power distribution system based on 480 VAC/120 VAC distribution copper/iron-steel transformers and corresponding copper or aluminum power lines and cables.
Thirty years ago, the equivalent of today""s desktop computer filled an entire office room, and yet had only a fraction of its computing power. This tremendous reduction in size, weight, energy consumption, and especially cost has not yet occurred in the field of power electronics, energy distribution, power conversion and control. The reason for this is that the silicon technology that led the computer industry to revolutionize the world has not yet been seriously applied to the copper-and-iron realm of power engineering. However, this is now a possibility through the application of two new technologies: Cryogenic Power Conversion (CPC) using Low-Temperature Operated Semiconductor Devices (LOTOS) and High-Temperature Superconductors (HTS). The basic research for CPC has been done already and awaits implementation.
The concept of Cryogenic Power Conversion is based on the fact that the conduction losses of power MOSFET switches (1) and the switching losses of IGBTs, MTOs, Thyristors, GTOs, etc. (2) are reduced by cooling the devices to the temperature of liquid nitrogen (77K). The first statement is confirmed by the on-resistance measurements of FIG. 1 for the MOSFET APT10026JN (1000 V, 33 A, 0.26 xcexa9). The area between the 300 K and 77 K curves represent the Cryo-Gain which is even larger taking into account the higher junction temperatures (400-425 K) in actual operation.
FIG. 2 shows the typical temperature dependence of the same conduction-loss producing parameter. The improvement factor for the on-resistance reduction between 375 K and 77 K is F=35, certainly an impressive number.
The physics behind this effect is the drastic increase at low temperatures of the majority carrier electron mobility in the drain-drift region of a high-voltage n-channel power MOSFET. MOSFETs are the fastest switching power devices available (1). Even further improvements, i.e. further reductions in on-resistance, are possible using the new Cool-MOS power devices developed by Siemens (3-5) and recently also by International Rectifier Corporation. Cryo-MOSFETs are best for applications below 1000 V. In minority carrier devices such as IGBTs, IGCTs, IEGTs, MTOs, MCTs, etc. for the higher voltage range (1 kV-6 kV) (6) charge storage limits the switching speed. These charges are proportional to the minority carrier lifetime which in turn is drastically reduced by cryo-cooling thus reducing switching times, and herewith also the switching losses considerably. FIG. 3 shows how the resistive (100 xcexa9 load) turn-off time of a 1700 V IGBT is reduced from 288 ns at 300 K to 39 ns at 77 K when operated at 1200 V.
FIG. 4 shows the concept of Cryogenic Power Conversion. The power dissipation of a high power circuit is reduced at the source by cryo-cooling thus permitting a drastic size, weight and cost reduction by eliminating big heatsinks, etc.
The U.S. Department of Energy (DOE) is already funding several efforts to develop high-temperature superconducting cables. An added benefit of the proposed system is that only relatively short cable lengths are required, and these do not have to be designed for high-voltage ( greater than 100 kV) applications. Thus, the HTS industry finds a market for immediate implementation, and the entire system can be realized in a period of a few years for any new high-rise building or other large structure to be constructed. One can assume that sooner or later, such lossless cables will be available for applications in Cryogenic Energy Distribution. HTS cables will solve the key problem of CPC and the Cryogenic Energy Distribution System (CEDS), the cryo-cooling for the distributed cryo-power electronics by providing a cryogenic fluid such as liquid nitrogen. Cooling with liquid gases such as LN2 is the only practical solution for HTS cables today. Also, even if it should turn out that HTS cables are too expensive (7), the CED system could be implemented using high-purity liquid-nitrogen-cooled cooper or aluminum cables, whose DC resistance drops by about the same factor (xcx9cxc3x977-8) that applies to LN2 generation (7-10 W/W). This improvement factor is much higher in very pure, but more expensive copper and aluminum cables.
Another advantage of cryogenic operation is the drastic reduction of the thermal conductivity of silicon and the usual substrate materials such as beryllium-oxide, etc.
In summary, it is believed that the prior art does not interfere with this disclosure. None of the referenced patents teaches the intricate combination of high-temperature superconducting cables with high-efficiency cryo-cooled silicon power electronics in a large-scale system. In addition, prior art represented by conventional transformers is totally different from the new technology proposed in this patent application. The conventional electrical power distribution system is basically about hundred years old, is not very efficient and did not yet profit from the many new technologies such as high-temperature superconductors and the whole semiconductor technology with its tremendous potential for size, weight and cost reduction at increased reliability.
The object of this invention is to provide an electrical power/energy distribution system which is more efficient than the existing one by using two new technologies thus saving a considerable amount of energy if implemented on a large scale in big cities. It will also reduce global warming.
The existing electric distribution infrastructure has remained unchanged for almost a century. A key component of this system is the so-called xe2x80x9cdistribution transformerxe2x80x9d converting 480 VAC, 60 Hz into 120/240 VAC, 60 Hz. This transformer has a relatively low efficiency of xcx9c95% and has been called by experts the xe2x80x9cweak linkxe2x80x9d in the US energy distribution system. Accordingly, this invention introduces two new technologies to change this situation providing the benefits of higher efficiency, reduced energy consumption, and in addition power load shedding, voltage regulation, improved power quality, power factor correction and control. Additional benefits are possible and will be discussed.
The new Cryogenic Energy and Power Distribution (CED) system according to this invention is shown in FIG. 5 based on the application of the two new technologies of High-Temperature Superconductivity (HTS) and Cryogenic Power Electronics (CPE). The CED system combines the following concepts, features and configurations:
A central high-temperature superconducting DC cable (650 VDC or higher) cooled by liquid nitrogen (LN2, 77 K) is installed and extends from the top floor to the basement of the high-rise (or any other) building through its center (FIG. 5).
HTS Cable Power Distribution: High-Temperature Superconducting (HTS) DC cables supply electrical power and the cooling mediumxe2x80x94liquid nitrogenxe2x80x94to the main power loads. The use of direct current permits a doubling of the transmitted energy per wire cross-section and the elimination of AC losses in the HTS cables, thereby allowing for reduced cable weight. The DC voltage is 650 VDC (480 VAC rectified), 2xc3x97325 VDC or any other suitable voltage (1.2 kV, 2.4 kV).
Cryogenic Power Conversion (CPC): The conventional heavy and bulky copper/iron core transformers (for example 460 VAC/120 VAC, 75 kVA: 500 pound, 1-2 square meter footprint) are replaced by ultra-small, lightweight DC/AC solid-state inverters or DC/DC converters made with cryo-cooled power MOSFETs or other suitable semiconductor power devices. Their size and weight may be in the order of magnitude of a few liters or kilograms respectively. These cryo-silicon transformers (650 VDC to 120/240 VAC) may be integrated directly with the HTS cable structure due to their small size, exhibit an electrical efficiency of  greater than 99.8% (without cooling penalty), and they save a lot of valuable, expensive rental space.
With some minor modifications, the cryo-transformer/inverter circuits can also be used as motor drives and adjustable speed drives (ASDs) in factories requiring many motors. They have a high overdrive capability: A few seconds of 300 kVA power can be obtained from a 100 kVA inverter.
Cryogenic Power Electronics (CPE) provides drastically increased reliability and lifetime. In addition, it permits improved power quality, power factor correction, voltage regulation, HTS current limiting, load shedding and so on.
A capacitive energy storage system based on cryo-cooled ceramic capacitors can easily be integrated into the CED system providing additional power quality.
The thermal insulation of the circuit dewar and the HTS cables can be implemented using polymer thin-film multi-layer isolation (MLI), which can also be used to increase the capacity of a capacitive energy storage system (See U.S. application Ser. No. 09/593,196). The thermal losses of HTS cables or Cryo-Inverters can be dimensioned in such a way that they add to air-conditioning and space cooling in hot areas if desired. In this case the cables can be made at a lower cost.
The ubiquitous availability of liquid nitrogen in a CED system provides an excellent new means for fire protection and suppression. LN2 does not burn and drives away oxygen, and was therefore used to extinguish the burning oil wells in Kuwait! No more exploding oil transformers generating fires! Evaporated LN2 also does not generate any xe2x80x9cwater damagexe2x80x9d. This is one of the many advantages of the present invention.
The proposed CED system can immediately be used for commercial energy and power distribution in all new large buildings, especially high-rise buildings, factories, large-scale offices, ships, etc. The John Hancock building in Boston, for example, has 62 floors and 124 large and bulky cooper-iron distribution transformers requiring a special room for their installation. The implementation of this system also provides the feature of electrical power load shedding of 7-15%, depending on the air-conditioning load (outside temperature), and an electrical energy saving of 5-12% in high-rise city blocks. Additional savings are possible in these high-rise buildings due to the small footprint of the silicon transformer compared to conventional copper/iron transformers. Rental costs range from $200 to $600 per square meter per year. Thus, the rental value of one square meter of space for a 50 year time span is about $10,000 to $30,000. This price is much higher than the cost of a power transformer itself. The cryo-silicon transformer technology can also be applied to motor drives, especially in combination with HTS motors.
The complete CED system is shown in FIG. 5. The Cryo-Silicon Transformers are cooled by LN2, which is supplied by the HTS cable. It is assumed that each floor requires at least 1-2 inverters/transformers. At the top of the building, a liquid nitrogen tank is placed, as shown in FIG. 6. The HTS cable supplies the electrical power via 650 VDC obtained after rectification of the usual 3-phase, 480 VAC power source delivered by a conventional or HTS transformer from a high voltage source such as 13.8 kV or a fuel cell. Synchronous Cryo-MOSFET rectification can also be used in order to reduce the losses of the rectifier circuitry. Note that DC cables permit the transmission of twice the power for the same voltage ratings and wire cross-sections. Of course, other voltages could be used.
EPRI, the Electrical Power Research Institute, Palo Alto, Calif., issued a technical report in 1995 entitled xe2x80x9cProof of Principle of the Solid-State Transformerxe2x80x9d (8). It is believed that, if the silicon transformer is feasible at room temperature as shown by EPRI, it will be even better at cryogenic temperatures (77 K) in combination with HTS cables.
For a 60 Hz silicon transformer the PWM (pulse-width-modulated) switching frequency of a switch-mode inverter can be low (1-10 kHz) so that switching losses are also small or negligible if in addition soft-switching techniques are applied. The efficiency is determined by the ratio of on-state voltage and voltage swing. The APT MOSFET (Advanced Power Technology""s Metal-Oxide Silicon Field-Effect Transistor) APT 10050 LVR, rated 1000 V, 21 A, and 0.5 xcexa9 (at 300 K), has an on-resistance of 24.2 mxcexa9 at 77 K, i.e. immersed in liquid nitrogen (LN2). For a supply of 650 V, a current of 10 A, and 2 MOSFETs in series (as is usual in bridge circuits), the on-state voltage to voltage swing ratio is:   L  =            2      ⁢                        0.242          ⁢                      xe2x80x83                    ⁢          V                          650          ⁢                      xe2x80x83                    ⁢          V                      =    0.00075  
This corresponds to an efficiency of  greater than 99.9%. Assuming a cooling penalty of a factor 10 (10 W/W for LN2 production) and negligible switching losses at theses low frequencies an overall transformer efficiency of  greater than 99.0% should be possible. By paralleling more MOSFETs one can reduce the losses even more down to any desirable level: xe2x80x9cSilicon is cheapxe2x80x9d and silicon chips are small and have low weight! Considering xe2x80x9cLoad Sheddingxe2x80x9d one can use the 99.9% figure, since the cooling LN2 can be produced at a time of low load power consumption.
A main advantage of the cryo-silicon transformer is its potential for greatly reduced size, weight and cost. Nothing beats semiconductors as far as cost, weight and size reduction are concerned! A cryo-silicon transformer (FIG. 6) which does not require a large LN2 tank or an expensive cryo-cooler can be quite small. It can be put into a container of a few liters. Where a 75 kVA conventional copper/iron transformer weighs about 500 lbs, a cryo-silicon transformer may have a weight of a few pounds given the availability of a central LN2 source. LN2 is supplied by the HTS cable. It is believed that a cryo-silicon transformer can be made so small that it can be mounted inside the wall or floor of a high-rise building, thus saving much expensive space. FIG. 7 compares a conventional copper/iron transformer with a cryo-silicon transformer inverter.
The circuitry inside the cryo-inverter/transformer box shown in FIG. 6 and FIG. 7 can be implemented by conventional half-, full-bridge, 3-phase-bridge or a xe2x80x9cOpposed Currentxe2x80x9d full bridge topology (Stanley, PESC 1997). The latter is shown in FIG. 9.
It should be obvious that the system according to this invention can also be applied in several high-rise buildings which are close together so that the same cryo-cooling system can be used for all of the buildings: FIG. 8. In this case an air liquefication plant could be placed right into a central location of this city block as shown.
The fire suppression system coordinated with the energy distribution structure is shown in FIGS. 10-12.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and the ensuing description.